This invention generally relates to sealed collets wherein the slots between the collet fingers are filled with a bonding agent to maintain a flow of pressurized coolant through the interior of the collet. It is specifically concerned with an improved sealed collet having agent-free gaps in the collet slots for increasing the radial flexibility of the collet body, which in turn facilitates the removal of the collet from the locknut of a chuck assembly.
Collets for use in tool-holding chuck assemblies are well known in the prior art. Such collets generally comprise a tubular body formed from a plurality of elongated, flexible steel fingers. The fingers are separated by longitudinal slots that impart some degree of radial flexibility to the collet which allows it to selectively grip the shank of a rotating tool such as a drill bit. Adjacent gripping fingers are interconnected by an alternating pattern of metal webs to form a one-piece collet body. In operation, the collet body is inserted in a complementary-shaped opening in a chuck shaft so that its distal end projects out of the shaft. An annular locknut having an inner diameter screw thread that matches an outer diameter screw thread on the shaft is then installed over the shaft and distal end of the collet body. The locknut has a nose ring with a frustro-conical cam surface that engages the distal end of the collet body and squeezes it radially inwardly as the locknut is screwed onto the chuck shaft. The radial compression that the locknut applies to the distal end of the collet body flexes the body inwardly, creating a gripping force between the inner diameter of the collet body and a tool shank inserted therein.
Sealed or bonded collets are also known in the prior art. In such collets, the slots separating the individual collet fingers are filled with a silicon-based rubber compound for two reasons. First, the filling of the collet slots with such a compound prevents pressurized coolant intended to flow through a bore in a gripped tool shank from flowing away from the shank and out through the walls of the collet body. Second, the bonding agent prevents the entry of dirt, metal swarf, and other debris from entering the collet slots and lodging between the shaft and the inner surface of the collet fingers. This function is important, as the presence of dirt or other debris in such locations can adversely affect the ability of the collet to uniformly grip the outer surface of the shank of the tool, which in turn can result in an off-center grip that might seriously compromise the accuracy of the bores or other cuts made by the tool.
While such bonded collets have been found to work well for their intended purpose, the recent use of coolants under higher pressure (i.e., on the order to 1000-1500 psi) has necessitated the use of harder, stiffer bonding agents to prevent the formation of leak paths through the collet slots. These stiffer bonding agents are less compressible, and adversely interfere with the radial compressibility of the collet. In the past, when the coolant pressures were lower (i.e., under 1000 psi) softer, more compressible bonding agents could be used to fill the slots of the body without any significant danger of leak paths occurring. The relatively more compressible bonding agents used in the prior art did not stiffen the radial resiliency of the collet to an extent where the collet body could not be readily compressed by the locknut to securely grip a tool shank. Even more importantly, the use of such pliant bonding agents would not render the distal end of the collet body so stiff as to interfere with the manual removal of the locknut if and when it became necessary to change the collet body to grip a shaft of a different size. Unfortunately, the use of stiffer bonding agents, such as one sold under the trademark SUPERFLEX 587 and manufactured by Loctite, Inc., located in Rocky Hill, Conn., has not only reduced the radial compressibility of the collet body to the extent to where manual compression around a shaft via the locknut is difficult, but has further made it very difficult for a system operator to manually separate the collet body from the locknut when a change of collet becomes necessary.
Clearly, there is a need for an improved collet body that is radially compressible to an extent to where the gripping of tool shanks and the removal of the locknut is easily performed by a system operator, but yet which can conduct high pressure coolant streams while preventing leakage through the collet slots. Ideally, such an improved sealed collet should not involve any redesign or machining of conventional collet bodies and locknuts, and should be installable onto existing collets (sealed or otherwise) with a minimum amount of effort and expense. Finally, such an improved sealed collet should still be able to resist the entry of dirt, debris, and metal swarf between the collet fingers and a gripped tool shaft.